Improvements in and relating to biofiltration

Abstract: Sod fuel peat is crushed in a pin crushing drum plant 3 which produces crushed sod turf which is screened to provide nodules of peat, at least 50%, and preferably approximately 72% w/w of the nodules having an average dimension of between 10 mm and 20 mm. The nodules have a bulk density at 50% moisture content of at least 400 Kg/m³. The nodules are a highly effective biofiltration medium.

Description: The invention relates to a biofiltration medium and in particular to a biofiltration medium based on peat. Biofiltration is a method of treatment for cleaning gas streams originating from a number of industrial and waste treatment activities. The removal of odorous components from the gas takes place by the activity of microorganisms (mainly bacteria) which are present on an organic carrier material or medium. Waste gases are passed via ducting into a space located beneath or above the filter medium. As the gas passes through the filter the odorous compounds are absorbed onto an aqueous layer and are degraded by microorganisms which reside within the organic filter material. The end-products of the oxidation process are odourless compounds, mainly carbon dioxide, water, nitrates and sulphates. Oxidation restores the absorption capacity of the filter material and provides the microorganisms with the nutrients necessary for their survival and growth. Thus the entire biofiltration process is self generating. One of the main advantages of biofiltration in comparison with other techniques is that many chemically different compounds can be eliminated with a high degree of efficiency due to the adaptation of the microorganisms to the compounds present in the gas stream. Biofilters require little maintenance and have low energy requirements. The running costs of biological odour treatment technologies have been found to be 2-10 times lower than for other treatment methods. As there are no secondary disposal problems, biofiltration is the most "environmentally friendly" option for odour control. Biofiltration is a proven odour control technology for a number of applications as described in the VDI guideline of biofilters 3477 (1991). Biofiltration of gases is a result of physical, chemical and biological processes. The main physical process are adsorption and absorption. Malodorous compounds are adsorbed onto the filter medium and absorbed onto the layer of water surrounding the medium. The chemical reactions that take place on the surface of the biofiltration medium include precipitation, ion exchange and chemisorption. The biological processes are the breaking down of waste gases by microorganisms. The efficiency of a biofilter depends on a large number of factors. One of the main factors is the biofiltration medium that is used in the biofilter. More particularly, the invention relates to a biofiltration medium comprising discrete units consisting of peat. EP-A-413638 describes a biofiltration medium of this type in which the discrete units of peat may be balls or pellets consisting of a mixture of a fragmented organic material and a binder. The binder consist of humidified peat classes 3 or 4 on the Modified von Post Scale. The fragmented organic material may be peat classed 1 or 2 on the Modified von Post Scale. In this case the ratio of dry mass of humidified peat to the mass of said fragmented organic material is between 0.7 and 3.9. The fragmented organic material may also be sawdust in which case the value of the ratio of dry mass of humidified peat to the mass of sawdust is 0.3 to 0.5. The invention is characterised in that the biofiltration medium comprises peat nodules formed from high density fuel peat having von Post number of from 6 to 10, the nodules having a bulk density of 50% moisture content of at least 400 kg/m 3 and at least 50% (w/w) of the nodules having an average dimension of between 10 mm and 20 mm. In a preferred embodiment of the invention, at least 60%, most preferably 70% and especially approximately 72% (w/w) of the nodules have an average dimension of between 10 and 20 mm. Preferably less than 10% of the nodules have a dimension greater than 20 mm. In a preferred embodiment of the invention less than 15% of the nodules have a dimension between 5 and 10 mm. In a particularly preferred embodiment of the invention, the bulk density of the nodules is at least 450 and preferably greater than 475 Kg/m 3 . In a particularly preferred embodiment of the invention, high density fuel peat from which the nodules are formed has a density from 400 to 500 Kg/m 3 . Preferably the peat from which the nodules are formed is well decomposed sod peat or lump peat. The invention also provides a method for producing a biofiltration medium comprising crushing high density fuel peat to produce crushed sod turf and screening the crushed sod turf to produce the peat nodules. Preferably the method includes the steps of:- screening the crushed sod turf to provide a fines fraction, an on-size fraction, and an over-size coarse fraction; eliminating the fines fraction; chopping the coarse fraction; and screening the chopped coarse fraction to provide further on-size fraction peat nodules having the desired density and size range. In a preferred embodiment the crushed sod turf is screened in a double deck screen base having two screens. Preferably to produce the desired size range the screens are 22mm and 12 mm screens. In one embodiment of the invention oversize fraction from the double deck screen base is recycled for chopping and subsequent re-screening. The invention will be more clearly understood from the following description thereof given by way of example only with reference to the accompanying drawing which is a schematic view of a method for producing a biofiltration medium according to the invention. Specialised high density well decomposed sod (lump) fuel peat is tippled into a receiving hopper 1 and delivered by a conveyor 2 to a conical pin crushing drum plant 3 which produces crushed sod turf. The fines from the crushed sod turf are extracted and discarded. The coarse size fraction is delivered by a conveyor 4 to a double deck finger screen box 5 fitted with 22 mm and 12 mm wire screens. On-size fraction 10 from the screen box 5 is the desired peat nodule product which is delivered by a conveyor 6 for storage. Fines 11 are eliminated. Oversize crushed sod turf that does not pass through the screen box 5 is recycled along line 7 to a flail/chopper unit 8 from which it is delivered again to the conveyor 4 for delivery to the screen box 5. Nodule Properties The peat nodules produced by this processing technique have the following distinctive properties for biofiltration purposes: Bulk Density (average) ≥ 475 Kg/m 3 (at 50% moisture) Moisture Content ≤ 37% Particle Size (w/w) 90% ≤ 20 mm 82% ≥ 10 mm 96% ≥ 5 mm The well decomposed sod fuel peat is highly degraded peat having von Post numbers of from 6 to 10. It is specifically selected to produce nodules which are characteristically hard and consequently resistant to abrasion and load deformation and which will undergo minimum expansion when absorbing water. The peat nodules according to the invention provide a suitable environment for microbial growth and create sufficient voids to achieve a low pressure loss, small specific filter resistance, low mass and low energy requirements for good drainage and oxygenation. The structure of the nodules is relatively regular and even resulting in uniform penetration by waste gases at a low pressure drop in biofiltration systems. The nodules provide a large surface area for physical, chemical and biological reactions to occur. They also provide good buffering against pH fluctuations in the material being treated in the biofilter. Because of the relatively low density of the nodules, they exhibit low back pressure characteristics and can therefore be used to construct biofilters of up to 3 metres high. EXAMPLE 1 Using the peat nodules described above, a biofilter was assembled with the following results. Application: Animal By-product Rendering Source of foul gas: Factory ventilation air Non-condensable process gases Pretreatment of non-condensable gases: Cyclones for removal of particulates Packed tower acid scrubber Air Flow Rate: 23,039m 3 /hr Biofiltration Medium: Nodules having the properties given above. Biofilter volume: 250m 3 Depth of Biofilter bed: 1m Chemical Analysis CHEMICAL INLET OUTLET Hydrogen Sulphide (ppm) 1.5 not detected Mercaptans (ppm) 0.5 not detected Ammonia (ppm) 7 not detected Amines Present + not detected Triethylamine (ppm) 32 not detected Olfactometric Analysis Biofilter Inlet 44,839 OU/m 3 Biofilter Outlet 158 OU/m 3 Inlet odour concentration (after pretreatment) 10,000 - 50,000 OU/m 3 Performance: Mean odour removal efficiency across filter bed Date % At installation 98.7 6 months after installation 98.5 9 months after installation 99.5 EXAMPLE 2 Application: Sewage Treatment Bed Size: 55m 3 at 3m depth Air Flow Rate: 5,500 3 /hour Chemical Analysis CHEMICAL INLET OUTLET Hydrogen Sulphide (ppm) 7.5 not detected Mercaptans (ppm) 2.1 not detected Ammonia (ppm) 2.0 <0.1 The invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail within the scope defined by the claims.

Claims: A biofiltration medium comprising discrete units consisting of peat characterised in that the biofiltration medium comprises peat nodules formed from high density fuel peat having a von Post number of from 6 to 10, the nodules having a bulk density at 50% moisture content of at least 400 kg/m 3 and at least 50% (w/w) of the nodules having an average dimension of between 10 mm and 20 mm. A biofiltration medium as claimed in claim 1 wherein at least 60% (w/w) of the nodules have an average dimension of between 10 mm and 20 mm. A biofiltration medium as claimed in claim 1 or 2 wherein at least 70% (w/w) of the nodules have an average dimension of between 10 mm and 20 mm. A biofiltration medium as claimed in any preceding claim wherein approximately 72% (w/w) of the nodules have an average dimension of between 10 mm and 20 mm. A biofiltration medium as claimed in any preceding claim wherein less than 10% (w/w) of the nodules have a dimension greater than 20 mm. A biofiltration medium as claimed in any preceding claim wherein less than 15% (w/w) of the nodules have an average dimension of between 5 mm and 10 mm. A biofiltration medium as claimed in any preceding claim wherein the bulk density of the nodules at 50% moisture content is at least 450 Kg/m 3 . A biofiltration medium as claimed in any preceding claim wherein the bulk density of the nodules at 50% moisture content is at least 475 Kg/m 3 . A biofiltration medium as claimed in any preceding claim wherein the peat from which the nodules are formed has a density of from 400 to 500 Kg/m 3 . A biofiltration medium as claimed in any preceding claim wherein the peat from which the modules are formed is well decomposed sod peat or lump peat. A method for producing a biofiltration medium as claimed in any preceding claim comprising crushing high density fuel peat to produce crushed sod turf and screening the crushed sod turf to produce the peat nodules. A method as claimed in claim 11 including the steps of:- screening the crushed sod turf to provide a fines fraction, an on-size fraction, and an over-size coarse fraction; eliminating the fines fraction; chopping the coarse fraction; and screening the chopped coarse fraction to provide further on-size fraction peat nodules having the desired density and size range. A method as claimed in claim 12 wherein the crushed sod turf is screened in a double deck screen base having two screens. A method as claimed in claim 13 wherein the screens are 22mm and 12 mm screens. A method as claimed in claim 13 or 14 wherein oversize fraction from the double deck screen is recycled for chopping and subsequent re-screening.

Inventors: BYRNE ANNE, BYRNE, ANNE
Applicants: BORD NA MONA
Application year: 1994
Main classification: B01D53
IPC codes: B01D53